1. Field of the Invention
The present invention relates to a polarization independent optical isolator and, more particularly, to a reflection type of a polarization independent isolator.
2. Description of the Related Art
Optical fiber communication systems are now in practical use, and efforts are being made to advance research and development in this field. Accordingly, requirements for optical devices with more versatile functions have also increased.
An optical isolator is used as a functional component in a light transmission system such that light transmission therethrough is permitted in only one direction. A common use of optical isolators is as constituents of so-called xe2x80x9coptical passive componentsxe2x80x9d within optical amplifier systems, which are themselves important components of fiber-optic communication systems. Optical amplifier systems generally include optical isolators residing on both sides of an optical gain element such as an Er-doped fiber. Other optical passive components include Wavelength Division Multiplexers (WDM""s) and signal monitors.
A polarization dependent optical isolator 100 is shown in FIG. 1 as an example of a traditional and typical optical isolator of the prior art. As illustrated in FIG. 1, there is provided a 45-degree Faraday rotation element (which is also referred to as a Faraday rotator) 101 which always rotates light input thereto in one direction by virtue of a permanent magnet. A polarizer 102 and an analyzer 103 are respectively placed before and after the Faraday rotation element, with the polarizer 102 and analyzer 103 being maintained at relative positions rotated 45 degrees with respect to one another.
As shown in FIG. 1, light emitted from an optical fiber 104 is divided or separated into parallel beams by a lens 105, and of the parallel beams, the polarizer 102 allows only polarized light oriented in a particular direction to pass through it; any other light is absorbed or reflected and eliminated. Polarized light that has passed through the polarizer 102 emanates from the Faraday rotation element 101 with its plane of polarization rotated by 45 degrees. The analyzer 103 is so arranged that polarized light with its plane of polarization rotated by 45 degrees passes through the analyzer 103, is focused by a lens 106 and enters an optical fiber 107.
On the other hand, and also as shown in FIG. 1, of light entering the polarization dependent optical isolator 100 in the reverse direction (from the optical fiber 107), only polarized light that is rotated by 45 degrees relative to the polarizer 102 may pass through the analyzer 103. Polarized light that has passed through the analyzer 103 will have its plane of polarization rotated by 45 degrees by the Faraday rotation element 101, and then emanates therefrom. The resulting light is rotated by 90 degrees relative to the polarizer 102 and is eliminated. Because of this, light in the forward direction propagates forwardly while light in the reverse direction is eliminated.
However, the isolator 100 just described is polarization dependent, even with respect to light propagating in the forward direction. More particularly, only specific, polarized light can pass through the isolator 100 in the forward direction, and the remaining propagating light is not effectively utilized because it is eliminated. Typical optical fibers used in light wave communication and data transfer systems do not preserve optical polarization over long distances. Light emanating from such a fiber consists of a randomly mixed state of light polarized in all directions, regardless of the state of polarization of light input to the fiber. Polarization-preserving fiber is well known but is too expensive for general use over long distances. Polarization independent optical isolators have therefore found a wide variety of applications in fiber-optic light wave systems.
FIG. 2A shows a well-known prior-art polarization independent optical isolator that is disclosed in U.S. Pat. No. 4,548,478. In the prior art polarization independent optical isolator 200 shown in FIG. 2A, tapered birefringent plates (tapered plates) 201 and 202 are placed on either side of a 45-degree Faraday rotator 203. Referring now to FIG. 2A, when light emanates from the optical fiber 204 into the prior art polarization independent optical isolator 200 and enters in the forward direction into the first tapered plate 201, the light is divided or separated into ordinary rays (o-rays) and extraordinary rays (e-rays) because of the differences in the index of refraction of the first tapered plate 201 due to polarization. These rays are refracted to different directions, and enter the 45-degree Faraday rotator 203 of FIG. 2A.
Ordinary and extraordinary rays of which planes of polarization are rotated 45 degrees by the Faraday rotator 203 are caused to enter the second tapered plate 202. The second tapered plate 202 is arranged such that an optical axis of the second tapered plate 202 is rotated 45 degrees around or about the light propagation direction relative to an optical axis of the first tapered plate 201. Therefore, the foregoing ordinary and extraordinary rays correspond to ordinary and extraordinary rays in the second tapered plate 202, respectively. Accordingly, ordinary rays and extraordinary rays that pass through the second tapered plate 202 emanate parallel to each other. These parallel beams of ordinary and extraordinary rays are focused onto the optical fiber 207 by the lens 206.
On the other hand, light traveling in the reverse direction (emanating from fiber 207 and traveling toward the direction of fiber 204 as shown in FIG. 2B) is divided into ordinary rays and extraordinary rays after entering the second tapered plate 202. The ordinary rays and the extraordinary rays are refracted to different directions by the second tapered plate 202, enter the 45 degree Faraday rotator 203, and are emitted therefrom with their plane of polarization rotated by 45 degrees.
For the light propagating in the reverse direction as shown in FIG. 2B, ordinary rays and extraordinary rays in the second plate 202 are converted to extraordinary rays and ordinary rays, respectively, in the first plate 201 by the Faraday rotator 203, so that the direction of each of these rays after passing through the first tapered plate 201 is different from that of incident light. Accordingly, when these rays are converged by the lens 205, focal points are formed outside the face of the fiber end 204 so that the light traveling in the reverse direction does not enter the optical fiber 204.
Since optical isolators typically utilize Faraday rotators and since the angular polarization rotation of Faraday rotators typically depends on wavelength of the light propagating therethrough, the wavelength region that provides the 45-degree rotation is very narrow. Therefore, a high isolation is maintained only in a very limited wavelength region, unless deviation from 45-degree rotation is compensated for.
In U.S. Pat. No. 4,712,880, two optical isolators and two polarization rotation compensators which are incorporated into these optical isolators are disclosed. The first polarization rotation compensator described in U.S. Pat. No. 4,712,880 is shown in FIG. 3A as element 300 and is composed of a combination of a half-wave plate 301 whose principal axis is inclined at an angle of xcex8/2 with respect to the plane of polarization of the incident light 302 and a quarter-wave plate 303 whose principal axis is inclined at an angle of xcex8 with respect to the plane of polarization of the incident light 302, with the half-wave plate 301 and the quarter-wave plate 303 disposed in this order with respect to the forward light propagation direction.
The second polarization rotation compensator described in U.S. Pat. No. 4,712,880 (not shown) is similar except that the principal axis of the quarter-wave plate is parallel to the plane of polarization of the incident light, the principal axis of the halfwave plate is inclined at an angle of xcex8/2 with respect to the plane of polarization of the incident light, and the quarter-wave plate and half-wave plate are disposed in this order with respect to the forward light propagation direction.
The first optical isolator described in U.S. Pat. No. 4,712,880 utilizes the first polarization compensator, and is shown in FIG. 3B. This first optical isolator 304 comprises a first birefringent wedge plate 305, the first polarization rotation compensator 300 described herein above with reference to FIG. 3A, a Faraday rotator 306, and a second birefringent wedge plate 307, all arranged in this order with respect to the direction of propagation of the forward light. The forward light emanates from fiber 308, and is collimated by lens 309 onto the first birefringent plate 305. After passing through the first birefringent plate 305, the first polarization rotation compensator 300, the Faraday rotator 306, and the second birefringent plate 307, the light is focused by lens 310 into fiber 311, as shown in FIG. 3B.
The second optical isolator described in U.S. Pat. No. 4,712,880 (not shown) is similar except that the second embodiment of the polarization rotation compensator is used and the first birefringent wedge plate, the Faraday rotator, the second polarization rotation compensator, and the second birefringent wedge plate are arranged in this order with respect to the propagation direction of the forward light.
The prior-art optical isolators discussed above are of the transmission type. Reflection-type optical isolators can reduce the number of optical components, because some components are used twice due to the double pass characteristics of the device. FIG. 4 is a perspective view of a prior art reflection-type polarization independent optical isolator that is disclosed in U.S. Pat. No. 5,033,830. As shown in the prior art reflection-type polarization independent optical isolator 400 of FIG. 4, a pair of stacked reciprocal rotators 401 and 402, namely half-wave plates, a Faraday rotator 403, and reflector 404 (including lens 404-1 and mirror 404-2) are positioned in tandem adjacent to the birefringent plate 405. In the forward (transmitting) direction, a light wave signal exiting an optical fiber 406 is split into a pair of orthogonal rays by the birefringent plate 405. The orthogonal rays then pass through a first reciprocal rotator 401 and the Faraday rotator 403 for rotating polarizing light planes. The Faraday rotator 403 rotates polarizing light planes 22.5 degrees. The rotated rays are then redirected by the reflector 404 back through the Faraday rotator 403. After passing through the second reciprocal rotator 402, the orthogonal rays re-enter the same birefringent plate 405 where they are recombined and launched in an output fiber 407.
Since a Faraday rotator 403 is a non-reciprocal device, any signal traveling through the isolator in the reverse (isolation) direction will be split on both passes through the birefringent plate 405 such that neither will intercept the input fiber 406.
A second prior-art reflection-type polarization independent optical isolator suitable for use as an optical passive component in an optical amplifier is disclosed in U.S. Pat. No. 5,499,132, incorporated herein by reference, and is shown in FIGS. 5A and 5B. The second prior-art reflection-type polarization independent optical isolator 500 shown in FIGS. 5A and 5B includes at least two optical fibers 501 and 502, an optical fiber array 503 into which the fibers are secured and whose tip end is polished at the angle of approximately 8 degrees, a birefringent crystal 504 for dividing the input light into two linearly polarized lights, a half wave plate 505 for reversibly rotating the direction of polarization of the input light by approximately 45 degrees, a graded index type rod-lens 506 to collimate and focus the light, a magnetooptical crystal 507 and associated magnet 508 for non-reversibly rotating the direction of polarization of light transmitted therethrough by 22.5 degrees counter clockwise on each pass, a reflector 509, and a glass plate 510.
In the second prior-art reflection-type polarization independent optical isolator 500 shown in FIG. 5A, the signal lights output from the first optical fiber 501 are divided by the birefringent crystal 504 into two linearly polarized light rays, which are then collimated by the lens 506. Thereafter, the planes of polarization of the two linearly polarized light rays are rotated by xcfx80/8+nxcfx80/2 (n=0, 1, . . . ), respectively, in, for example, the left-hand direction by the magnetooptical crystal 507. After reflection by the reflector plate 509, the two linearly polarized light rays each receive a further rotation of xcfx80/8+nxcfx80/2 (n=0, 1, . . . ) in the same direction by the magnetooptical crystal 507. A further rotation xcfx80/4 in the same direction is caused by the half wave plate 505. Thereafter, a polarized light coupling operation is effected by the birefringent crystal 504 so as to input the light rays into the second optical fiber 502. Thus, the above described arrangement functions as a polarization-independent optical isolator.
Also disclosed in U.S. Pat. No. 5,499,132 are additional embodiments illustrating extended function of the optical isolator disclosed therein so as to provide an integrated set of optical passive components for use in an optical amplifier. An example of such an embodiment is shown in FIG. 5 of U.S. Pat. No. 5,499,132 in which provision is made for injection, in a direction counter to the signal propagation direction, of two 1480 nm laser-diode pump beams into the optical path as well as for detection and monitoring of a portion of the amplified signal. FIG. 5 of the ""132 patent is reproduced herein as FIG. 6, for the convenience of the reader.
The above described prior art optical isolators are all of the single-stage type-that is to say that light inputted thereto passes at most one time through the full set of isolation-providing components. Generally, one stage of isolation structure provides an isolation characteristic of about 35 dB. Therefore, the prior art polarization independent optical isolators described above, although useful for many applications, have an insufficient isolation characteristic for applications to the high quality transmission systems or the optical fiber amplifiers.
In U.S. Pat. No. 5,689,360, incorporated herein by reference, a double-stage reflection isolator is disclosed. FIG. 1 of U.S. Pat. No. 5,689,360 discloses a device comprising both first and second optical isolation units and also a reflection unit for coupling the output of the first optical isolation unit to the input of the second optical isolation unit by directing the outputted signal ray from the first optical isolation unit to the second optical isolation unit, with the signal ray received by the first optical isolation unit transmitting in the opposite direction to the outputted signal ray from the second optical isolation unit. FIG. 1 of the ""360 patent is reproduced herein as FIG. 7, for the convenience of the reader, using the reference numerals provided in the ""360 patent. The operation of each of the single stage isolators disclosed in U.S. Pat. No. 5,689,360 is similar to that disclosed in U.S. Pat. No. 5,499,132 (and is not repeated in detail here) except that the reflector is not incorporated within each isolator. Instead, the reflector element is positioned after the output of the first optical isolator and before the input of the second optical isolator, with respect to the forward light propagation direction, so as to provide optical coupling between the two isolators. The sequential operation of two optical isolators in this fashion provides improved isolator performance with respect to the operation of a single-stage isolator.
An object of the present invention is to provide a single-stage polarization independent optical isolator with improved isolation characteristics.
Another object of the present invention is to provide a single-stage broadband polarization independent optical isolator with improved performance characteristics.
An additional object of the present invention is to provide a double-stage polarization independent optical isolator with improved isolation characteristics.
A further object of the present invention is to provide a double-stage broadband polarization independent optical isolator with improved isolation characteristics.
Still another object of the present invention is to provide an isolator/monitor/amplifier.
Another object of the present invention is to provide an optical system based upon the isolator/monitor/amplifier of the present invention.
Yet a further object of the present invention is to provide an optical system based upon the double-stage broadband polarization independent optical isolator and the isolator/monitor/amplifier.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The present invention is a polarization independent isolator and an optical system based thereon. The polarization independent isolator of the present invention includes a single stage polarization independent isolator, a single stage broadband polarization independent isolator, a double stage polarization independent isolator, and a double stage broadband independent isolator. The present invention also includes an isolator/monitor/amplifier, and respective optical systems based upon the isolator/monitor/amplifier of the present invention and on the isolator/monitor/amplifier in cascade with the double-stage broadband polarization independent isolator of the present invention.
The single stage polarization independent isolator of the present invention comprises an input fiber; an output fiber; optical elements including a birefringent walk-off plate, a counterclockwise rotating xcex/2 plate provided adjacent thereto, and a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and the counterclockwise rotating xcex/2 plate; a lens; and a mirror. Input light traveling in the forward direction from the input fiber passes through each of the above-mentioned optical elements and enters the output fiber. However, the above-mentioned optical elements prevent input light from the output fiber traveling in the reverse direction from entering the input fiber.
The single stage broadband polarization independent isolator of the present invention comprises an input fiber; an output fiber; optical elements including a birefringent walk-off plate, a counterclockwise rotating xcex/2 plate and a broadband polarization rotation compensator provided adjacent thereto, and a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and the counterclockwise rotating xcex/2 plate and broadband polarization rotation compensator; a lens; and a mirror. Input light of many wavelengths traveling in the forward direction from the input fiber passes through each of the above-mentioned optical elements and enters the output fiber. However, the above-mentioned optical elements prevent input light from the output fiber traveling in the reverse direction from entering the input fiber.
The double stage polarization independent isolator of the present invention comprises an input fiber; an output fiber; optical elements including a birefringent walk-off plate, a counterclockwise rotating xcex/2 plate provided adjacent thereto, a clockwise rotating xcex/2 plate provided adjacent to the birefringent walk-off plate and to the clockwise rotating xcex/2 plate, a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and the counterclockwise rotating xcex/2 plate and the clockwise rotating xcex/2 plate, and a second birefringent walk-off plate; a lens; and a mirror. Input light traveling in the forward direction from the input fiber passes through each of the above-mentioned optical elements and enters the output fiber. However, the above-mentioned optical elements prevent input light from the output fiber traveling in the reverse direction from entering the input fiber.
The double stage broadband polarization independent isolator of the present invention comprises an input fiber; an output fiber; optical elements including a birefringent walk-off plate, a counterclockwise rotating xcex/2 plate and broadband polarization compensator provided adjacent thereto, a clockwise rotating xcex/2 plate and second broadband polarization compensator provided adjacent to the birefringent walk-off plate and to the clockwise rotating xcex/2 plate and broadband polarization compensator, a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and to the broadband polarization compensator and the clockwise rotating xcex/2 plate, and a second birefringent walk-off plate; a lens; and a mirror. Input light traveling in the forward direction from the input fiber passes through each of the above-mentioned optical elements and enters the output fiber. However, the above-mentioned optical elements prevent input light from the output fiber traveling in the reverse direction from entering the input fiber.
The isolator/monitor/amplifier of the present invention is based upon the single stage broadband polarization independent isolator of the present invention, and further includes a laser input and monitor output.
An optical system of the present invention includes two of the isolator/monitor/amplifiers of the present invention coupled to each other in series through an Er-doped fiber or other suitable optical gain element. The two isolator/monitor/amplifiers of the present invention coupled in series replace the optical passive components of a prior art optical system.
In addition, the present invention is a cascaded optical system including a double sided broadband polarization independent optical isolator of the present invention coupled in series to an isolator/monitor/amplifier of the present invention. Input signal light makes two passes through the optical system, being output from the isolator/monitor/amplifier along Er-doped fiber after the first pass therethrough. The Er-doped fiber then carries the input light back to the isolator/monitor/amplifier and the double sided broadband polarization independent isolator for a second pass through the cascaded optical system.
Because of its function described herein, a xcex/2 plate is also referred to as a reciprocally rotating optical element.